Spectrally resolved images are widely used to identify features based on their spectral content. The most familiar approach is three-color imaging, which is used for visual identification. Hyperspectral imaging is an extension of color imaging, in which images are collected in a large number of contiguous spectral bands; it is widely used to detect features in an image based on spectral structure. Typically, a full spectrum is recorded for each spectral element in a two-dimensional scene to form a three-dimensional hypercube containing spatial and spectral information. The hypercube data is typically stored for later analysis and detection.
Spectral filtering has long been used to enhance specific features in an image. Most filtered spectral imagers employ a single, broad, band pass that enhances the features of interest. For example, hot combustion sources can be imaged using a band pass around 4.2 microns corresponding to hot carbon dioxide.
The current invention belongs to a class of devices that produce spectrally encoded images by first dispersing the light from a distant scene, encoding the light spectrally using a spatial mask that selects specific combinations of wavelengths, and then recombining the light on a detector that records the spectrally encoded polychromatic signal. Such devices often make use of a programmable spatial light modulator, such as a digital micromirror array, to create the spatial mask. The programmable modulator provides the ability to encode an arbitrary spectral pattern on each image element. The pattern can be used to generate a specific and highly complex spectral band pass on an image. Spectra can then be obtained by cycling the system through a sequence of spectral band passes.
It is known to use a sequence of spectral band passes to record spectra using Hadamard Transform spectroscopy. This approach uses a sequence of orthogonal spectral pass bands for spectral encoding, followed by decoding in a post-processing computer.
The use of digital micromirror devices for spatial encoding in spatial light modulator spectrometers is taught by Stafford (U.S. Pat. No. 5,504,575), and Fateley (U.S. Pat. No. 6,046,808). Both employ a single input element and a single output element, defined by either a fiber or a detector. Sweat et al. (U.S. Pat. No. 6,504,943) employ an input slit and linear array detectors to encode a spectral pass band on a one-dimensional image. This patent also teaches the use of spectral matched filters to identify objects in a scene.
Tague (U.S. Pat. No. 5,923,036), MacKentry (1999) [MacKentry, J. W. and the NGST-MOS Study Team. “NGST-MOS A Multi-Object Spectrometer using Micro Mirror Arrays Final Report of the NGST-MOS Pre-Phase A Science Instrument Study of the NGST Project” Final Report NASA contract NAS5-98167 (1999)], Gentry (U.S. Pat. No. 6,996,292), and Fateley (U.S. Pat. No. 6,859,275) teach methods of spatial-spectral imaging in which a spatial light modulator is used to define the input slit of a spectrograph, allowing either a single detector spectrograph or a one-dimensional imaging spectrograph to select different spatial elements of an input image for spectral analysis.
Fateley (U.S. Pat. No. 6,859,275) also teaches a wide variety of devices using spatial-spectral information processing. All use only a single pass through a dispersive element, and do not recombine the light to form a polychromatic image. All imaging devices are based on spectrally filtering an active source.
Hyperspectral Hadamard imaging spectrometers are known, for example as taught by Wuttig et al. 2002 (Wuttig, A., and Riesenberg, R., “Sensitive Hadamard Transform Imaging Spectrometer with a simple MEMS,” SPIE vol. 4881, (2002)), Wehlburg et al. 2001 (Wehlburg, C. M., Wehlburg, J. C., Gentry, S. M., and Smith, J. L, “Optimization and characterization of an imaging Hadamard spectrometer,” Proc. SPIE Vol. 4381, p. 506-515, Algorithms for Multispectral, Hyperspectral, and Ultraspectral Imagery VII, Sylvia S. Shen, Michael R. Descour, Eds., (2001)), and Gentry et al. (U.S. Pat. No. 6,996,292). These devices produce full two-dimensional images using a two-dimensional array detector. The input image is passed though two spectrographs, one to disperse the image, and one to recombine the spatially encoded image onto the two-dimensional detector array. The spectral bandpass of each pixel is defined by the spatial mask overlaying the dispersed image and the detector elements. The use of the detector elements, rather than a separate entrance slit to define the bandpass differentiates these devices from other SLM spectrometers, such as Tenhunen (U.S. Pat. No. 6,870,619) and Brooks (U.S. Pat. No. 5,815,261).
Some spectrographs have a curved convex grating dispersive device. Many hyperspectral imagers, such as those taught by Chrisp (U.S. Pat. No. 5,880,834) and Reininger (U.S. Pat. No. 6,100,974) have used a single spectrograph, along with a slit and a two dimensional array detector, to produce a hyperspectral imager with one dimension of spatial resolution and one dimension of spectral resolution. Most such hyperspectral imagers use the traditional lateral Offner configuration, in which the input and output beam are displaced from the center of curvature, and light is dispersed along the displacement axis. This results in good imaging performance along the length of the slit, which is positioned perpendicular to the dispersion axis. The imaging performance is poor however along the dispersion direction, making this approach difficult for two-dimensional imaging applications.
Wuttig et al. 2002, Wehlburg et al. 2001, and Gentry et al. (U.S. Pat. No. 6,996,292) apply two such lateral Offner spectrographs to disperse and recombine a two-dimensional image. The spectrographs are highly modified from the original Offner configuration in order to improve image quality along the dispersion direction.